Method of atomizing molten magnesium



United States Patent 3,520,718 METHOD OF ATOMIZING MOLTEN MAGNESIUM JohnN. Reding, Midland, Mich., assignor to The Dow Chemical Company,Midland, Mich., a corporation f Delaware No Drawing. Filed Aug. 3, 1967,Ser. No. 658,037 Int. Cl. B22d 23/08; C23c 1/00 US. Cl. 117131 5 ClaimsABSTRACT OF THE DISCLOSURE In producing alloy pellets by the atomizingmethod disclosed in US. Pat. No. 2,699,576 by allowing molten metal tofall freely in a thin stream onto a steel disc or wheel rapidly rotatingin a non-reactive gas atmosphere, heretofore, only magnesium base alloyscontaining from 0.025 to 1.0 percent by weight zirconium and from 0.25to 7.0 percent by weight zinc could be so atomized.

As disclosed in the above patent, magnesium base alloys containing nozirconium or zinc, and magnesium base alloys containing less than theminimum amount of either zirconium or zinc, or both, as required by US.Pat. 2,699,576 cannot for the most part be successfully atomized usingthe method of said patent to obtain a uniform, quality, pelletizedproduct. The disc in these cases is substantially unwetted by the moltenmetal resulting in a non-uniform pellet product mixed with largeirregular agglomerates which are flung off the rotating disc.

A principal object of the present invention, therefore, is to provide animproved process whereby magnesium and essentially all magnesium basealloys can be atomized by the method as disclosed in US. Pat. 2,699,576.

An improved method of atomizing has now been found wherein the above andother objects and advantages have been found obtainable by adherentlycoating a clean ferous metal atomizing disc, whose surface to be coatedis essentially free of oxides, with certain metals as hereinafterdescribed.

In practicing the present invention, therefore, a ferrous metal disc,for example, as used in U.S. Pat. No. 2,699,576 for atomizing isinitially adherently coated, using essentially any coating means ortechniques, with a thin layer of a metal selected from the groupconsisting of cadmium, copper, nickel, tin and preferably zinc.Thereafter, the so-coated disc is used for atomizing magnesium and/ormagnesium base alloys, without regard for the limitation with respect toobserving minimum amounts of zinc and zirconium as was heretoforerequired and set forth in US. Pat. 2,699,576.

The so-coated wheel is initially wetter by the magnesium base alloybeing atomized, with continued wetting by said alloy of the underlyingferrous metal surface now exposed by dissolution of the coating metal.Dissolution of the :superstrate coating off the atomizing disc as itrotates by the molten metal intended to be atomized usually takes fromabout 5 to about 20 minutes.

By adherently coating the atomizing disc initially with a thin layer ofat least one metal of the group cadmium, copper, nickel, tin, or zinc,essentially all magnesium ice base alloys as well as primary magnesiumitself can be atomized to provide a visibly uniform pelletized producthaving a particle size substantially no coarser than about 20 mesh (US.Standard Sieve Series) and an essentially spherical shape.

Essentially any ferrous metal atomizing disc described in the aforesaidpatent can be treated as set forth herein to accomplish the objects ofthe invention and to prepare a resulting uniform pellet producttherefrom. The surface or surfaces of the disc intended to be employedin atomizing prior to coating with one or more of the coating metalsshould be relatively clean in that it should be essentially free ofgrease, dirt and the like and free of the various oxides usuallyappearing on ferrous surfaces due to corrosion, etc. The method ofcoating the ferrous disc surface is not limited other than by therequirement that the coating so-applied should reasonably be tenaciousand adherent. Any conventional or other method of coating or depositionmay be employed in the improvement of the present invention such asgalvanizing, electroplating or chemical plating, and the like. Asaforesaid, essentially all the iron oxides and other foreign materialthat may be present on ferrous metal surfaces should be removedtherefrom either before or during the coating operation, as ingalvanizing, such that no intermediate layer of oxide or other foreignmaterial exists between the coating metal and ferrous metal.

The thickness of the metal coating applied on the ferrous disc surfaceis not critical, except that a continuous coating should be obtained.Normally said coating will be thin and only a few mils (thousands of aninch) in thickness. No advantage results in applying a thick coating ofmetal and only results in a disadvantage of requiring a longer time tobe dissolved away by the molten coating metal during the first minutesof the atomizing operation.

The following examples serve to further illustrate the application ofthe present invention.

The following magnesium base alloys having the composition indicated,the balance of each composition being essentialy magnesium, wereprepared.

(1) 0.17% zinc+0.17% calcium and 0.4% zirconium, (2) 3.0% aluminum+l.0%zinc and 0.2% manganese, (3) 1.1% zinc+0.2% Misch metal.

In the molten state each melt was atomized by dropping a thin continuousstream thereof onto the center of a zinc galvanized, slightly concavedsurface of a 4 inch diameter steel disc, the major axis of which washorizontally disposed. The disc was rotating at about 10,000 r.p.m. in anon-reactive gas atmosphere. The underside of the disc was partiallythermally insulated so that the upper surface could be maintained at atemperature slightly above the melting point of the particular alloybeing atomized.

The disc was initially wetted by the molten metal. During continuationof the atomizing operation the initial zinc coating on the disc wassubsequently dissolved away. Essentially no problems were encountered ascontinued wetting of the disc by the molten alloy and atomizing of thealloy into uniform solidified pellets resulted by contact of the metalbeing atomized with the exposed clean disc surface. The molten alloy wasflung off the rotating disc whereupon the small droplets quicklysolidified into uniform spherical pellets with a minimum of fines.Essentially all the pellets so-produced were no coarser than about 20mesh (US. Standard Sieve). After the atomizing of each melt wascompleted and the disc cooled, a wettable solidified coating of thealloy re- 1 A commercial mixture of rare earth metals containingpredominant amounts of cerium, neodymium. the balance comprising smalleramounts of other rare earth metals.

mained on the atomizing surface of the disc such that the disc could besubsequently used again without an initial coating operation ashereinbefore described.

COMPARATIVE EXAMPLES In order to further illustrate the beneficialresults obtained by the improvement of the present invention, additionalmelts of the above alloys were prepared and atomized similarly asdescribed above except that the ferrous atomizing disc was not coated byany coating metal. The molten metal atomized only slightly andirregularly wetted the disc surface. The pellets produced were generallynon-uniform and irregular in shape. Moreover, a large portion of themolten magnesium base alloys was flung off the rotating disc inrelatively large masses or blobs which did not solidify prior to impactwith a shield surrounding the atomizing disc. Rather, the materialbecame virtually plastered into large accumulations on said shield whichwould periodically fall off to render the pellet product collectionexcessively non-uniform.

The results of employing an atomizing disc not coated in accordance withthe invention when compared to that obtained as illustrated in Examples1, 2 and 3 above, clearly show the great benefit and utility of theimprovement in atomizing of the present invention.

The present invention may be modified and changed without departing fromthe spirit and scope thereof and the invention is only limited asdefined in the appended claims.

I claim:

1. In a method of atomizing molten metal in which a thin stream ofmolten magnesium or magnesium base alloy falls upon the upwardly facingsurface of a rapidly rotating, heated, steel disc to fiingoff said metalas molten droplets which solidify upon being expelled from the disc toproduce atomized pellets therefrom; the improvement comprising coatingsaid disc surface intended to contact the molten metal to be atomizedwith a thin adherent continuous coating of a coating metal selected fromthe group cadmium, copper, nickel, tin or zinc prior to contacting saiddisc with said molten metal to be atomized.

2. The improvement of claim 1 wherein the disc is coated with a thinadherent coating of zinc.

v3. The improvement of claim 1 wherein the disc is coated with a thinadherent coating of cadmium.

4. The improvement of claim 1 wherein the disc is coated with a thinadherent coating of tin.

5. The improvement of claim 2 wherein the disc is coated with zinc bygalvanizing.

References Cited UNITED STATES PATENTS 2,699,576 1/1955 Colbry et a1.264-8 2,912,346 11/1959 Kanter l17131 X 3,300,298 1/1967 Reding 117--131X ALFRED L. LEAVITI, Primary Examiner J. R. BATTEN, JR., AssistantExaminer US. Cl. X.R.

